Menu:

Version:

June 12, 2010:
Revised: v2.0

ZL2PD TuneAid

A little device to help tuning up those
transmitters and antenna tuners

Introduction

After I built my KN-Q7 kitset 40m QRP single sideband (SSB) transceiver, described elsewhere on this site, I went on to
build a matching low power "Z-match" antenna tuner and a sensitive SWR
meter. For some reason, I found I needed three hands to tune this
apparently simple arrangement – One hand to hold the PTT button down,
and then two more hands to simultaneously adjust the controls on my
antenna tuner, while simultaneously whistling into the microphone, after delivering
the essential voice station identification, of course!

For those new to ham radio, an audio tone or a reasonably continuous
level of speech is required into the microphone so an SSB transmitter
will generate some RF power. That RF power allows the SWR meter to
measure the match between transmitter and antenna, and determine when
the antenna tuner is correctly adjusted.

My problem didn’t stop with the lack of three hands. A really bad cold
had left me with a decided lack of "whistle-power". Those who suffer
from asthma can probably appreciate this latter problem all too well.

Adding a tone oscillator into the KN-Q7 QRP
transceiver would have solved the problem, but space inside that radio
is very limited. This external TuneAid oscillator was my solution.

The Design

The TuneAid is a simple combination of a timer and audio tone
oscillator. It's a 100% discrete parts design, built with common small
signal audio PNP and NPN transistors. The circuit diagram is shown in
Figure 1.

Pressing
pushbutton SW1 turns on Q1. This then turns on the oscillator (Q3 and
Q4). With C1 initially discharged, Q2 is turned hard on, keeping
Q1 turned on after pushbutton SW1 is released. So, only a brief press
of the power-on switch is required to start the tone generator.

C1 slowly charges through R4, steadily reducing the voltage on the base of Q2. After about 10 seconds,
Q2 turns off, turning Q1 off, and of course, this then turns off the oscillator. Diode D1 ensures C1 is
rapidly discharged for the next time around. Battery drain in this
standby state is minimal, ensuring excellent battery life.

The Q3/Q4 low component count audio oscillator is an arrangement often
used for morse code practice oscillators. A small 8 ohm speaker
generates a surprisingly loud tone level, and this is held close to the
transceiver microphone during the tuning process.

Figure 1: TuneAid Circuit Diagram

Building the TuneAid

The PCB is shown in Figure 2 and the component layout in Figure 3. It
measures 36 x 20 mm (1.4” x 0.8”)

The prototype was built into a small plastic box just large enough to
hold the PCB, speaker and 9V battery. I used one similar to Jaycar’s
part number HB-6015. I milled out the speaker slots on the box cover to
match the speaker. The speaker was a flat 25mm diameter type recycled from an old cordless phone,
but any 8 ohm speaker, large or small, will work fine.

Figure 2: TuneAid PCB

All other components are readily available from the usual sources, the
transistors being any small signal types such as a BC548 (NPN) or BC559
(PNP). C2 is a mylar capacitor, but a disc ceramic type will work perfectly
well in this circuit.

My pushbutton (Jaycar part
number SP-0622) has a bright blue LED built in, and it looks
great while the TuneAid is operating. However, this is a relatively expensive
part to use for such a simple switch. Feel free to substitute any cheap pushbutton you can
find.

Figure 3: TuneAid PCB Component Layout

Using the TuneAid

While I still need three hands to manage the tuning process, I don’t
have the whistle problem any more. A quick press of the TuneAid button
will give me a 10 second tone burst, easily long enough to manage the
tuner, and the PTT key.

The battery has lasted for almost 12 months of
intermittent use, and it is still going strong.